Rank-1 ambiguity DOA estimation of circular array with fewer sensors

The uniform circular array (UCA) is usually considered to be free of ambiguity (rank-1) in DOA estimation. However, it is shown in this paper that the UCAs with sensors less than or equal to six are of rank-1 ambiguity (except for M = 5). In addition, we propose an effective scheme to construct the rank-1 ambiguity free non-uniform circular array (NUCA) with six sensors. Simulate results demonstrate that the proposed NUCA is free of rank-1 ambiguity in a wide frequency band with respect to the ratio of circular array radius to wavelength ranging from 0.5 to 3.